1. Field of the Invention
The present invention relates to an unsaturated polyester resin composition for the production of an onyx-like artificial marble, which comprises an unsaturated polyester resin and aluminum hydroxide filled therein.
2. Description of the Related Art
It is known that an artificial marble is prepared by filling at least one member selected from inorganic compounds having a refractive index close to that of an unsaturated polyester resin, such as aluminum hydroxide, alumina white, calcium aluminate and glass powder, into the unsaturated polyester resin.
An especially high transparency can be obtained if aluminum hydroxide is used, because the refractive index of aluminum hydroxide is 1.57 and very close to that of an unsaturated polyester resin, the product is relatively cheap, and the quality is stable. Accordingly, aluminum hydroxide is most widely used for the production of artificial marble products. Proposals have been made to specify the particle size and average particle size for obtaining a resin molded body having an improved transparency. For example, Japanese -25 Unexamined Patent Publication (Kokai) No. 55-27306 teaches that aluminum hydroxide having a particle size in the range of from 0.5 to 200 .mu.m, an average particle size of 8 to 60 .mu.m and a whiteness of at least 92 (as measured by a powder whiteness meter) is filled, and Japanese Unexamined Patent Publication (Kokai) No. 58-189215 teaches that aluminum hydroxide comprising a first group of particles having an average particle size of 10 to 30 .mu.m and a second group of particles having an average particle size of 50 to 90 .mu.m is filled.
Aluminum hydroxide prepared according to the Bayer process generally contains organic impurities unavoidably included from the preparation process, and therefore, if this aluminum hydroxide is filled, the obtained resin molded body is tinted a light yellow color or light reddish brown color. Even if decoloration is performed by using a pigment or the like, a clear color cannot be manifested, and this aluminum hydroxide is not suitable for obtaining products for which a beautiful color is required, such as a washhand stand, a polymer bathtub, a counter and the like.
The above-mentioned problem of coloration can be avoided if the whiteness is improved in aluminum hydroxide by removing organic impurities in the Bayer process by a special means, and this aluminum hydroxide is filled. However, in view of sedimentation in the state of a slurry to be filled in the resin and the strength, surface smoothness and stain resistance of the molded body, aluminum hydroxide having an extremely fine particle size should be used, and in this case, the transparency of the molded body is unavoidably sacrificed. Moreover, even according to the above-mentioned proposals to specify the particle size and average particle size of aluminum hydroxide, the transparency is still insufficient, and development of an unsaturated polyester resin molded body having an improved deep transparency is still desired.
As a means for obtaining a molded body having an excellent transparency, there can be mentioned a method in which a vitreous frit is used as a filler. However, since the vitreous frit contains a strongly alkaline component, the boiling resistance of the molded body is especially poor, and the composition cannot be used for the production of a molded body for which a high water resistance and boiling resistance are required, for example, a bathtub or the like.
Under this background, it is a primary object of the present invention to provide an unsaturated polyester resin having a reduced coloration and an improved deep transparency, and having an excellent water resistance and boiling resistance.